The present invention relates to a magnetic brush development apparatus for use in electrophotographic copying machines, electrostatic recording apparatus or the like.
In a magnetic brush development apparatus, magnetic developer is attracted to a non-magnetic cylindrical sleeve having inner magnets, to form a magnetic brush on the surface of the non-magnetic sleeve, and either the non-magnetic sleeve or the inner magnets are rotated, so as to transport the magnet brush formed on the sleeve to a development area where the magnet brush is brought into contact with a latent electrostatic image formed on a latent electrostatic image bearing member, for development of the latent electrostatic image. Usually, the magnetic brush is transported by rotating the non-magnetic sleeve, while the magnets are fixed. As the developer for use in magnetic brush development, a two-component type developer comprising magnetic carrier and toner, or a one-component type developer consisting of magnetic toner, is employed.
In such magnetic brush development apparatus, it is preferable that development be performed while mixing the developer sufficiently so as to be in a loose state. In order to attain this, at least two rotatable, non-magnetic sleeve having inner magnets are employed by disposing one over the other, whereby the developer particles are sufficiently mixed and dispersed when the developer is scooped up by the lower non-magnetic sleeve and delivered to the upper non-magnetic sleeve. In a magnetic brush development apparatus of the above-mentioned type, the magnets within the two sleeves located in the area where the developer is transferred from one sleeve to the other sleeve are usually opposite in polarity. According to experiments conducted by the inventors of the present invention, in the above-mentioned type magnetic brush development apparatus, with the magnets arranged in the above-mentioned manner, when the two sleeves are rotated at a comparatively low speed, the developer can be sufficiently transferred from the lower sleeve to the upper sleeve. However, as the rotation speeds of the two sleeves are increased for high speed development, the performance of developer transportation from the lower sleeve to the upper sleeve decreases. The result is that the developer deposited on the lower sleeve is not transported to the upper sleeve, but remains on the lower sleeve, bringing about an insufficient supply of the developer for development, reduction of image density and non-uniform development of images.
The above-mentioned phenomenon may take place for the following reason: In the above-mentioned type magnetic brush development apparatus, once the developer is deposited on the lower sleeve, the developer is comparatively strongly attracted to the lower sleeve, since the magnet in the lower sleeve and that in the upper sleeve in the developer transition region are opposite in polarity to each other. However, when the two sleeves are rotated at a high speed, the developer deposited on the lower sleeve can stay there, but readily it could not happen that the developer is detached from the lower sleeve and clings to the surface of the upper sleeve which is rotated at a high speed.